Metric system overhaul will dethrone the one, true kilogram

General science interest story. Likely to generate puns. I will keep mine to myself.~ctm

By Adrian ChoNov. 6, 2018 , 4:05 PM The atoms in a sphere of silicon-28 were counted to fix the Avogadro constant and redefine the mole. A copy of Le Grand K, the kilogram standard, can be seen in the sphere’s reflection. PTB

Like an aging monarch, Le Grand K is about to bow to modernity. For 130 years, this gleaming cylinder of platinum-iridium alloy has served as the world’s standard for mass. Kept in a bell jar and locked away at the International Bureau of Weights and Measures (BIPM) in Sèvres, France, the weight has been taken out every 40 years or so to calibrate similar weights around the world. Now, in a revolution far less bloody than the one that cost King Louis XVI his head, it will cede its throne as the one, true kilogram.

When the 26th General Conference on Weights and Measures (CGPM) convenes next week in Versailles, France, representatives of the 60 member nations are expected to vote to redefine the International System of Units (SI) so that four of its base units—the kilogram, ampere, kelvin, and mole—are defined indirectly, in terms of physical constants that will be fixed by fiat. They’ll join the other three base units—the second, meter, and candela (a measure of a light’s perceived brightness)—that are already defined that way. The rewrite eliminates the last physical artifact used to define a unit, Le Grand K.

The shift aims to make the units more stable and allow investigators to develop ever more precise and flexible techniques for converting the constants into measurement units. “That’s the beauty of the redefinition,” says Estefanía de Mirandés, a physicist at BIPM. “You are not limited to one technology.” But even proponents of the arcane changes acknowledge they may bewilder nonexperts. “Cooler heads have said, ‘What are we going to do about teaching people to use this?’” says Jon Pratt, a physicist at the U.S. National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland.

The new SI generalizes the trade-off already exploited to define the meter more precisely in terms of the speed of light. Until 1983, light’s speed was something to be measured in terms of independently defined meters and seconds. However, that year, the 17th CGPM defined the speed of light as exactly 299,792,458 meters per second. The meter then became the measurable thing: the distance light travels in 1/299,792,458 seconds. (The second was pegged to the oscillations of microwave radiation from cesium atoms in 1967.)

The new SI plays the same game with the other units. For example, it defines the kilogram in terms of the Planck constant, which pops up all over quantum mechanics. The constant is now fixed as exactly 6.62607015×10-34 kilogram meters squared per second. Because the kilogram appears in that definition, any experiment that previously measured the constant becomes a way to measure out a kilogram instead.

Such experiments are much harder than clocking light speed, a staple of undergraduate physics. One technique employs a device called a Kibble balance, which is a bit like the mythical scales of justice. A mass on one side is counterbalanced by the electric force produced by an electrical coil on the other side, hanging in a magnetic field. To balance the weight, a current must run through the coil. Researchers can equate the mass to that current times an independent voltage generated when they remove the mass and move the coil up and down in the magnetic field.

The real trickiness enters in sizing up the current and voltage, with quantum mechanical devices that do it in terms of the charge of the electron and the Planck constant. Now that the new SI has fixed those constants, the balance can be used to mete out a slug with a mass of exactly 1 kilogram. The redefinition also effectively makes the quantum techniques the SI standards for measuring voltages and currents, says James Olthoff, a NIST physicist. Until now, the SI has defined the ampere impractically, in terms of the force between infinitely long current carrying wires separated by a meter.

But applying the complex new definitions will baffle anybody without an advanced degree in physics, argues Gary Price, a metrologist in Sydney, Australia, who used to advise Australia’s National Standards Commission. In fact, he argues, the new SI fails to meet one of the basic requirements of a units system, which is to specify the amount of mass with which to measure masses, the amount of length with which to measure lengths, and so on. “The new SI is not weights and measures at all,” Price says.

The funny thing is, I don’t think they CAN measure it to that precision. It’s the STANDARD, not a requirement for measurement. It behooves others to meet the standard. It is not a requirement of the standard to bend to common use.

Having said that, it remains to be seen whether they could design circuitry to compensate for the mentioned imprecisions. My guess is that today, the answer is no.

It isn’t easy but it can be done and it’s independent of reference frame which is why is why they have done it. The reality is they will still probably produce physical standard bars in exactly the same way as they do now so for the average country with there standards organization it means nothing.

So it doesn’t matter that its hard or that you can’t do it yourself you buy one from a standards organization or buy scales that have been calibrated to the standard … AKA from you perspective nothing has changed so go back to sleep.

Yes! South Africa in the 70’s went nuts about using SI exclusively. As Atlas Aircraft Corporation’s (and S.A’s) one-and-only Mass Properties Engineer, I had running battles with the country’s Standards Authority, who would not allow me to ‘weigh’ an aircraft or its components (as required by international standards). So, I had to define in the glossary ‘Weighed” means to take an item or aircraft, place it upon “mass measuring equipment” and record the reading. Gah!

Probably should add if you really want to see standards drift go to China and buy 10 cheap rulers then put them all together on a desk. I guarantee you that by eye you can see the 300mm or 12inch marks don’t all line up that the printing or etching process have slightly scaled things.

That is not at all what they are asking you to do they are asking you to weigh it against an approved SI standard or on some device that has been calibrated to an SI standard. That is so when I take your 1kg thing you sold me and I weigh it in my country it still weighs 1Kg because we are using the same standard.

I suspect your definition probably was a little longer and you didn’t ever understand the issue.

“Weighed” means to take an item or aircraft, place it upon “mass measuring equipment that has been calibrated to SI standards” and record the reading.

Actually, Crispin, it was a case of “duelling standards”. I had to produce an aircraft “Weight and Balance Manual” to international civil aviation standards (back in 1970’s). And one of the absolute requirements was that it must state the “weighed weight” of the aircraft to a defined state. I.e, not the one that we’d calculated, or predicted – the ACTUAL. But all ‘official’ publications had to be validated by an arcane set of government offices, including, for some reason, the SA Bureau of Standards. Many, many years later, I met a similar trouble producing a user manual for some software written for the Air Force. Bureaucracy continues – whatever the engineering form!

But applying the complex new definitions will baffle anybody without an advanced degree in physics,

I think that the concept of mass is baffling anyway if you think about it for a while. If e = mc^2, then mass is something strange, beyond understanding other than the measurement of it. Mass may convert to energy, but the idea that “stuff” converts to “motion” is beyond comprehension. Well, I think it is beyond comprehension.

I think that the concept of mass is baffling anyway if you think about it for a while.

Yes. It is.

Mass depends on velocity, but compared to what? Distance depends on our understanding of the speed of light in a vacuum (according to SI units). Since velocity depends on the observer, surely distance and/or time depend on the observer too? And all the miriad forms of energy, which can also be represented as mass, surely. Which depends on velocity. Which depends on time. And distance. And the observer!

The only thing that we can be sure of is that time and space both depend on the observer’s movement in restaurants, according to the well-researched theory of bistromathics. Everything else is most likely an illusion, probably caused by imbibing too much in said restaurants…

I am so sorry I left my fingerprints on that damned kilo ball when I last tried to clean it, messing everything up and changing its weight. I also apologized for using Windex and a toilet tissue to clean it. My bad. Sneezing on it did not help, I admit.

All of these standards are of course with respect to an observer stationary with respect to the measuring apparatus. And on the Earth. Or maybe it’s wrt to the stationary mass. Or maybe in a gravity-free vacuum? Or…? The mind boggles.

I should add it also aligns to Quantum mechanics correctly so you can port measurements in QM directly back to Classical measurements. From a QM perspective that just leaves temperature to be fixed up so we don’t have to make a group of QM statistics to match what classical physics measures as temperature.

Basically the kinetic energy (movement) of the particles in the thing you are measuring. The problem comes in QM there are many sorts of vibrations and rotations that can’t be measured in a classical sense. So you can send energy into the Quantum domain and bring the energy back into the classical domain and the energy appears to come from nowhere or vice versa, have it in the classical domain and make it disappear. You aren’t breaking any laws energy is still conserved just you can’t measure it in the classical world and we just do some weird quantum setups.

It leads to strange situations where you can have negative temperatures colder than absolute zero (https://en.wikipedia.org/wiki/Negative_temperature) you have to say it is that because it is still pulling heat from something that is already at 0K. Really all we are doing is playing Quantum word games because temperature is a classical construct.

So temperature is much like a rainbow it has meaning to humans in a classical observation sense but it is an observation of a set of circumstances not something fundemental. If you like it is more a “rule of thumb” based on human experience it is totally incomplete and you can’t build anything fundemental from it, and why it doesn’t exist in QM except as a grouping of statistics.

I define Kelvin from quantum mechanics of fermions. I use electron because it is the most common fundamental particle in the universe. I define Kelvin using the Fermi temperature of electron at 1 m de Broglie wavelength. Fermi temperature is the temperature when fermions transition from classical thermodynamics to quantum mechanics. It occurs when interparticle distance (D) is less than the de Broglie wavelength (y)

Since time is the reciprocal of frequency, it is relatively easy to derive a time standard from a frequency standard. A standard clock comprises a frequency standard, a device to count off the cycles of the oscillation emitted by the frequency standard, and a means of displaying or outputting the result.

Good job the passing of time doesn’t vary does it (maybe it does but how would we know?)

In bistromathics, time is not a constant. It’s proportional to when your waiter appears and when the food appears divided by the interval between the last bite and the moment your waiter understands that you want the check.

Apparently you’re not familiar with the Theory of Relativity, which states:

The more relatives that gather in one place, the slower time will be. If you get enough relatives together in one place, time will actually stop. Whether this is a good thing or a bad thing is relative to the observer.

It will never catch on . . .
there are those in society who are wedded to the old agrarian / pagan system of weights and will not change.
(although now linked to the metric system, but they try not to mention that)

Interesting article, however, more useful for laymen is to know the reasons for the sizes in the metric system:

Meter was defined as 1/10 000 000 of the distance between the North pole and Equator.Kilogram is the weight of a liter, i.e. a cubic decimeter, of pure water. One ton is the weight of on cubic meter pure water.Kelvin has the same size as Celsius, one 1/100 of the difference between freezing water and boiling waterMole – The weight of one mole substance in grams is the same as the atomic numberCandle – The light produced by a standard candle. One of the best-known of these was the English standard of candlepower. One candlepower was the light produced by a pure spermaceti candle weighing one sixth of a pound and burning at a rate of 120 grains per hour.

I thought it was the combined atomic weight in grams. The atomic weight never seems to equal an integer, either, just fairly close to one, and often just less than an integer. I never quite got that bit, because I assumed the electrons had some mass.

The atomic weight of Carbon 12 is exactly 12, the atomic weight for all other atoms and molecules have some decimals.

It is several reasons for this, one is as you say, the electrons also have some weight, another is that both protons and electrons weigh a little more than one atomic mass unit. The third reason is that the energy bound in the nucleus have some mass. This can be derived from Einstein’s famous equation E=mc*c, or m=E/(c*c)

The number of molecules in a mole is known as Avogadro number. If you take that number of similar molecules, the weight in gram is equal to the atomic weight of the molecule.

It is true that one atomic weight is one-twelfth of an atom of carbon-12. The atomic weights in the periodic chart are due to the natural occurrence in nature of the various isotopes of the elements. They do not factor out the isotopes of each element; that’s why the weights aren’t whole numbers. Some atomic weights are in parentheses. That means the element has no stable isotopes and the number in parentheses is the atomic weight of the isotope with the longest half-life.

But it’s the major reason. The electron masses are so tiny, that they don’t figure much in the masses per atom. And although the binding energies are large, the equivalent mass is also tiny per atom (E = m*c^2). The differences are mostly due to various isotopes and their natural ratios on the Earth. (A periodic table on another planet would have slightly different atomic weights, because the isotope ratios would or should change from planet to planet and star system to star system.)

Nope, whether or not electrons have mass, the fractional atomic weights arise from the average composition having different isotope abundances. Same number of protons, determining the element, but differing number of neutrons. 12C, 13C, 14C is an example. So Atomic mass of carbon = 12.0107.

But even more importantly, they got the meter WRONG – and by a LOT. It wasn’t even CLOSE to the definition you provide (which is the correct STATED definition). Personally, I love the metric system and welcome this improvement. But I also.love the “English” system and prefer it when it is more useful. It is based not primarily on the decimal system but on multiples of 2, and as my good mathematician friend used to love to say, there are 10 kinds of people in the world – those who understand binary and those who don’t.

Here is my favorite fact about US customary measures. An inch is 25.4mm exactly. A mile is 5280 feet of 12 inches each, which is 63,360 inches or 1,609,344 mm (1,609.344 meters). That is an international mile. But, in the 19th century, US law defined the foot as 1200/3937 meters, which is what all US land surveys were based on. So, a survey mile is (5280*1200)/3937 or 1,609.35 meters.

As the earth is not a sphere, your definition won’t work without specifying the circle that is your standard. According to NIST Handbook 44 Appendix C, the USA adopted a nautical mile standard of 1,852 meters exactly in 1954.

When navigating on a chart and wanting to find the distance between two points, you measure the distance with your dividers and read the distance off on the nearest longitude line. It’s close enough for government work.

>>
Let us hope the government has better processes and tools than that.
<<

We had INS (inertial navigation system–although the P-3 INS wasn’t all that great), LORAN, long-range TACANs, VORs, and sextant (if your were really hard up)–not to mention DR (dead-reckoning). Of course, the computer made it really easy to navigate.

Ocean navigation isn’t required to be that precise. But lets play the game. The circumference of the Earth at the Equator (a great circle) is 40,075 km. The pole-to-pole circumference is 40,007 km. Since we’re using longitude then the pole-to-pole distance is the one to use. Dividing 40,007 km. by 21,600 minutes in a circle (60*360), we get 1,852.176 meters. That’s about 0.0095% longer than your NIST value.

If we take a typical mission of about 3,000 nm. and ignoring all the other errors that might occur (wind, drift, etc.), we will be off by about 285 meters. Ask any navigator–being off by 285 meters after 3,000 nm. is nothing compared to the other errors you have to deal with.

>>
Mars_Climate_Orbiter
<<

That wasn’t exactly NASA’s greatest moment in space navigation. But only missing a target by a couple of hundred kilometers after 669 millions miles of travel is still quite a feat. However, they should have aimed high.

I like the metric system and it’s use of the decimal system.
What irritates me, old fogey that I am, is that it is declared as a ( apologies for the capitals] as a RATIONAL system.
The distance from equator from the North Pole was based on 1789 measurements. As accuracy improved it was found to be incorrect. Did the modest French change the length of the metre? No, they produced a metal bar.
Then the kilogram, based on on 1000 cubic centimetres of water. Until they realised that the purity and temperature of the water was significant. So they engaged an English firm of Johnson and Matthew to manufacture the Grand K.
My point is, that the pound and pint could have been established in exactly the same way. And if the Metric system is so logical why are things given a weight in kilograms rather than Newtons? Kilograms are a measure of mass, not weight. Weight relies on the local gravitational field.
For everyday use I still use miles, yards, feet, inches, pints and gallons. For DIY and plumbing for measurement below an inch the millimetre is much easier than fractions of an inch.
But amongst this triumphalism that new standards there is one thing bugging me. Where are relativistic effects taken into account?
The Earth spins from nearly 1000 mph to near zero. The Earth orbits the Sun at 60, 000 mph. The Sun is travelling through space at, (can’t be bothered to look it up), and our Galaxy is also travelling through space at some velocity.

Agree London. I have some problems defining a meter based on the “distance light travels in 1/299,792,458 seconds”. But the speed of light depends on the frame of measurement- the measurement and observer have to occur in the exact same frame of reference. Impossible to do.

Then mix in what happens in the presence of hyper density of mass- a black hole or a neutron star. The length of a meter apparently changes a LOT as gravity increases greatly. And gravity still has not been incorporated into quantum mechanics. E=mC^2 m=C^2/e C=sqrt(E/m).

So, much like Newtonian physics, however we measure, we can do it finely enough to make measurements useful under certain conditions.

I believe the manufacturers are Johnson Matthey, well known in mineral processing/metallurgical circles. One thing that bemuses me is that muricans refuse to adopt the metric system, but are quite happy to change the spelling of metre. One can deduce from this that I am readily bemused.

The Draft Resolution edited by the General Conference on Weights and Measures (CGPM), at its 26th meeting, can be found here: https://www.bipm.org/utils/en/pdf/CGPM/Draft-Resolution-A-EN.pdf.
I never met such radical reforms in physics! The Conference works more with fixed values while everybody knows these values always have a margin of uncertainty. Scientists are permanently struggling to sharpen these values.

“The Conference works more with fixed values while everybody knows these values always have a margin of uncertainty. Scientists are permanently struggling to sharpen these values.”

I don’t think this is fair to the people at Sevres or to measurement scientists in general. My own experience in working with such guys is that they are a lot more on the ball about uncertainty than many experimental scientists. They are fully aware of the various sources of uncertainty in making and using measurement standards.

The basic problem with the kilogram was that the mass of the primary standard, which we just called “K” (in a fancy typeface – never heard ANYONE call it “le grand K” – maybe they are just saying “capital k” in French?) was varying and that that variation, though tiny, meant that there was a limit to the accuracy with which any other mass could be measured using “K” as the standard while technical and scientific advances were requiring continually increasing accuracy.

What they are doing regarding the kilogram is pretty much what they have been doing with the metre. From 1889 to 1960 the primary standard of the meter was a metal (Pt/Ir) rod which was made as nearly as possible exactly the same length as the previous “metre des archives” of 1799*. In 1960 the definition was changed to a certain number of wavelengths of a particular emission of Krypton 86, further modified to the present definition based on the speed of light in 1983, to which is attached an agreed methodology – “the mise en pratique” for “realising” the metre. Both these last changes led to increases in the actual measurement precision obtainable and I’ve not heard of any problems arising.

On the other hand, when I was last looking at this topic I did wonder about one aspect of it. Using the Kibble balance as I understand it requires knowing the acceleration due to gravity at the location of the measurement and I’ve always been sceptical about whether precise enough measures of this are likely to be possible.

How fast was I going officer? Relative to what precisely? It was approximately 30,000 / 299,792,458 seconds, depending on season, time of day and direction of travel. Only taking into account Earth’s orbit ’round the sun of course, not the galaxy. Is that illegal or summin’?

Fortunately they haven’t changed standard unit of beauty in the Western world, which is still the millihelen, being the quantity of beauty necessary to launch a single ship.

Also, under the inverse Manhattan system used in newspapers, small distances are measured reciprocally, the standard of which is the the thinness of a human hair. (This supplanted the “thickness of a cigarette paper” when the tobacco scare gained ground.)

There is also the smallest distance known to science, which is the difference between the “freezing” and “scalding” settings on a specified Florida motel shower.

Because a pennyweight is a unit of the Troy system, used today only to measure precious metals, and then only in Troy ounces. While the scruple is a unit of the Apothecaries system, no longer used by pharmacists. The grain is the same in all Anglo-saxon customary unit systems, and is 64.7891 mg exactly.

I actually used the then alternative, now ‘official’ SI definition of the ampere in the derivation of my ‘new science’ intrinsic capacitance equation for Helmholtz double layer capcitance. Gave a physical quantity for a coulomb of charge, so a physical quantity interpretation of a farad of capacitance defined as a coulomb of charge at an electrical potential of one volt.
The equation, in turn, explained a lot of otherwise scientifically misunderstood experimental results, which in turn proved the fundamentally faulty misunderstanding of effective surface, the correction of which enabled my basic patents and their theoretical plus experimental validation.

If aliens found out that our unit of mass is defined by a lump of metal kept in a vault in Paris, we will be the laughingstock of the universe. The ideal definition of measurement units must be so fundamental that even aliens can understand them. If I were to define kilogram, second and meter, I will base them on the most common fundamental particle in the universe – the electron.

I define kilogram as a multiple A of electron mass m in kg. Where A = 1/m = 9.109 e31
Convert the electron mass to energy: E = m c^2 where c = speed of light
This gives a gamma ray with frequency: f = E/h where h = Planck constant
I define second as the frequency of the electron energy-equivalent: 1 second = 1.236 e20 cycles
The wavelength of this gamma ray: y = c/f
I define meter as a multiple B of wavelength y in meter. Where B = 1/y = 2.426 e12

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